Constant speed drive mechanism



Oct. 24, 1961 w. R. JOHNSON CONSTANT SPEED DRIVE MECHANSM Original FiledNov. 25, 1957 2 Sheets-Sheet 1 /N VEN TOP.- /A mf: P. Jams/:cw

ATTORNEYS Oct. 24, 1961 w. R. JOHNSON 3,005,940

CONSTANT SPEED DRIVE MECHANISM Original Filed Nov. 25, 1957 2Sheets-Sheet 2 Fla-2 TTOP/VEYS 3,005,940 CNSTANT SPEED @REVE MEEHANHSMWayne R. Johnson, Los Angeles, Calif., assigner to Minnesota'Mining &Manufacturing Company, St. Pani, Minn., a corporation of DelawareContinuation of appiication Ser. No. 698,627, Nov. 25, 1957. Thisapplication Feb. 2, 1959, Ser. No. 790,769 16 Claims. (Cl. $18-$05) Thisis a continuation of copending application Serial No. 698,627, filedNovember 25, 1957, by Wayne R. Johnson for Constant Speed DriveMechanism.

This invention relates to drive mechanisms for apparatus the speedwhereof must be maintained accurately at `a constant value when inoperation, suchy as precision recorders and reproducers of electricalwaves. While its utility is by no means limited to the example given therequirements of such apparatus are extremely rigorous and the inventionwill therefore be described specifically as employed for this purpose,its application to other apparatus having like requirements beingobvious to those skilled in the art.

In the recording and reproduction of electrical signals of certaintypes, such as television signals, radar signals, telemetering signalsand the like, the maintenance of constant speed'of recording media inboth recording and reproduction is extremely important. Variation inspeed results in angular modulation of the recorded waves that maydestroy the usefulness of the recording. Such variation may take theform of a more or less constant drift in the speed of the medium, frombeginning to end of the recording, or more frequentlyof oscillations inspeed around a constant value resulting in wow. In order to minimizesuch variations and reduce them to tolerable values types of. feedbackor servo circuits have been devised, most of which employ synchronous AC. drives. Such drives are bulky, heavy and expensive. In airborneequipment, in particular, they are undesirable in that the entire powerfor the drive must usually be developed from powerful stabilizedoscillators, with complicated servo circuits to control the frequency ofthese oscillators in the reproducing process'and to compensate forvariations in load upon the driving motor which would cause phasevariations in either recording or reproduction.

For many reasons Iit would be desirable to use a shunt type D.C. motoras the driver, because of its simplicity, lightness for a given poweroutput and reliability. The diiliculty has been that while such motorsare frequently referred to as constan-t speed devices, their speed infact varies with every variation in load and to compensate them hasrequired even more complex equipment than the use of synchronous drives.

Another requirement for. drives for mechanisms of the type herecontemplated is that they should not only be able to maintain a constantspeed of a desired value but that they should be able to attain thatspeed and pull into synchronism very rapidly. Many of the devicesintended to maintain constant speed are not only inactive during thepull-in period but may even inhibit the pull-in and require unduly longperiods before attaining the desired speed.

Among the objects of the present invention are, therefore, to provide adrive mechanism employing a direct current shunt-type motor and at thesame time to provide a precision speed control for such a motor thatwill not only maintain its average rotational speed at the desired valuebut that will also prevent deviations of instantaneous speed from theaverage to a high degree of accuracy, comparable to the best A.C.synchronizing systems, that will permit very rapid pull-in operation,bringing the drive to speed in less than a second, in the case of theparticular drive here described, and that in the case of a tape drivefor record- 3,005,940 Patented Oct. 24., 1961 ing and reproduction, isequally applicable to both recording and the reproduction processes.Another object, particularly applicable in cases of recording equipmentfor telemetering indications, is to provide a drive which is equallyeffective at many recording and reproducing speeds which are integrallyrelated.

Broadly considered, as indicatedl above, the apparatus of the presentinvention comprises a shunt-type D.C. motor, which forms the actualdrive means. Means are provided for generating a pilot wave, thefrequency whereof varies in direct proportion to the speed of the motor;for example, in a recording mechanism this can be a tachometer generatorwhose moving element is mounted directly on the motor shaft or, in thecase o f reproducing equipment, it may be arpilot wave imposed upon therecord from the output of such a generator. There is also provided meansfor developing'two waves of standard frequency, equal `to the frequencyof the pilot waves at the desired motor speed, whose fundamentalcomponents are in quadrature. While these may be sinusoidal waves theyare preferably of square waveform, but for convenience they will bereferred to as the sine wave and cosine wave irrespective of waveforms;the circuits to which they are supplied will be similarly referred to asthe sine and cosine circuits.

Up lto a certain point the sine and cosine circuits are identical; in apreferred embodiment, in which square waveforms are used, these wavesare first fed into an integrating circuit which converts the squarewaveform into a triangular one. From the integrating circuit both sineand cosine circuits connect to electronic switches, connected foroperation simultaneously, once in each cycle, by the pilot wave so thatthe switches close during a small fraction only of the standard wavecycle. Beyond the switches there is connected to each circuit a storagecondenser which charges to the potential of the respective circuit eachtime the switch is closed, retaining this charge until recharged (ordischarged) at the next cycle of the pilot wave.

Beyond the storage condensers the sine and cosine circuits differ. Thesine circuit connects to the input of a variable-gain D.C. amplifier,the output whereof supplies one of the two shunt circuits `'of the D.C.motor. Because of its lower inductance and hence its quicker response,it is usually the armature circuit of the motor that is supplied by theampliler. Preferably, too, the variable gain of the D C. amplifier isachieved through including in it a modulator stage, which controls thegain in accordance with the voltages Supplied by the cosine circuit aswill next be described.

The output of the storage condenser in the cosine circuit connects,usually through an amplifier, to a differentiating circuit whichadvances the phase of any alternating component in the storage condensercharge by degrees, thus converting the cosine component into a sinecomponent, which is applied to vary. the gain of the D.C. amplifier.

In operation the pilot wave closes the electronic switches in the sineand cosine circuits to sample waves existing therein simultaneously andowing to the quadrature relationship between the fundamentals of the twowaves a peak of one, positive or negative, coincides with a. zero of theother. In the situation where the motor is running at speedV theoperation stabilizes when the sampling occurs in the sine circuit on therising branch of the Sine wave, this being near the peak of the cosinewave. Aotually, at the operating point the sampling recurs atsubstantially constant phase of the standard wave; no leak is providedfor the storage condenser in this circuit and the D.C. amplifiertherefore holds the charge between operations of the sampling switch andprovides a constant drive for the D.C. amplifier. The latter has a highgain, although not the maximum vof which it is capable but more nearlythe middle of its range, as the cosine circuit is A.C. coupled and atoperating speed little or no A.C. signal'is present. The D.C. amplifieris so biased that in the absence of signals from the storage condenserit will supply enough current to operate the motor at some speed withinits operating range and so provide starting torque as soon as rotationstarts. The pilot wave immediately becomes active' to operate thesampling switches. It is Well recognized that a sampling operation is,in fact, a form of modulation, developing the -usual modulationproducts; The storage condensers effectively filter out the higherfrequency components, the major products remaining being the pilotfrequency and the difference frequency between the pilot frequency andthe standard frequency in the sine and cosine circuits, themodulationproductfrequencies appearing also in quadrature in the twocircuits. The differentiating network in the cosine circuit advances thephase of the frequency components in this latter circuit by 90' degrees.The phases'of the various frequencies in the two circuits, with respectto each other, depends upon whether the pilot frequency is greater thanor less than the standard frequency so that, referred to the signal inthe sine circuit, that in the cosine circuit, after being advanced 90degrees by the differentiating circuit, is either in phase or 180degrees out of phase. The cosine circuit is connected to the modulatingstage so poled that the sine components are additive if thepilotfrequeney is lower than the standard frequency and to subtract itif the reverse is the case. Accordingly, if the alternating componentsin the two circuits are' out of synchronism instead of being merely outof phase, a modulated component is added to the current supplied to themotor as a result of the normal bias, increasing the torque on the motorand tending to pull it into step, whereas if the speed of the motor istoo great the modulation components are applied in opposite phase,Vdecreasing the torque and bringing the motor back to the speed wherethe phase-control signal supplied by the sine circuit alone takescharge.

All of the above is more fully described in the detailed description ofa preferred embodiment of the apparatus which follows, this descriptionbeing illustrated by the accompanying drawings wherein:

FIG. 1 is a diagram partly in block form and partly schematic,illustrating the invention as applied to a magnetic tape recording andreproducing apparatus adapted to operate at any one of a plurality oftape speeds, those chosen for illustration being, respectively, 240inches, 120 inches, 60 inches, 30 inches and 15 inches per second; and

FIG. 2 comprises a number of curves illustrating the waveforms developedat various portions of the circuit of FIG.' 1.

The diagram of FIG. 1 illustrates the essential features of arnagnetctape recording and reproducing mechanism embodying the presentinvention. The particular apparatushere described is driven by 3/16 HP.,shunt-type D.C. motor whose eld winding 1 and armature 3 areschematically illustrated at the extreme right of the figure. The fieldis excited separately to a substantially constant value by any suitablevoltage source 5. The armature current is supplied from a D.C. amplifier7, the output of which is controlled by the balance of the equipmentshown. The armature 3i drives the magnetic tape record by means of acapstan' 9 mounted directly upon the motor shaft. The record tape andthe balance of the tape handling mechanism are not illustrated, sincethey are conventionaland the present invention lies in the drive and itscontrol, rather than the equipment driven, which is postulated merely togive a concrete example of the type cf'requirement that can be metthrough the use of the Invention.

In the case shown these requirements are extremely rigore-us. Tapespeeds of from 240" inches per second down to 15 inches per second, arat-io of 16:1 in 2:1 steps, must be maintained with complete accuracyas to synchronism and minimum deviation as to the phase of recorded andreproduced signals.

The necessary control to achieve this result is exercised by means of apilot wave whose frequency is directly proportional to the motor speed.In the recording process this wave is developed by a taehometergenerator mounted directly upon the motor shaft. While various types ofsuch generators are available, for the present purpose, where widevariation in frequency must be accommodated, it is preferable to use agenerator that will deliver a voltage independent of its speed ofoperation. One type having this property is that illustratedschematically in the ligure; a tachometer disc 11, having a circle ofequally spaced apertures 13 formed near its periphery is mounteddirectly upon the motor shaft. A light source 15, mounted on one side ofthe disc, illuminates a photocell 17, positioned on the other side ofthe disc, through the apertures, so that as the disc rotates a pilotsignal is developed in the photocell circuit whose frequency is theproduct of the rotational speed of the motor and the number ofapertures.

As will be developed later, the control exercised by this invention willhold the position of the armature, with respect to the norm, to within avery small fraction of a cycle of the pilot wave frequency. It isdesirable that there should be many cycles of the pilot wave perrevolution of the armature, for the higher the relative frequency of thepilot wave the more accurate the control. In the present instanceapertures are used in the tachometer disc and the size of the capstan issuch that at the highest tape speed of 240 inches per second the motorspeed is approximately 3600 rpm. or 60 revolutions per second. At thehigh speed, therefore, the tachometer develops a frequency ofapproximately 10 lrilocycles per second. Since the tape speed isdetermined by both the capstau diameter and the motor speed the exactspeed is not too important, a slight deviation of the motor speed fromthe assumed value can be compensated by an opposite proportional changein capstan diameter and therefore, for convenience in the description,it will be assumed that e the proper tape speed of 240 inches per secondis developed when the pilot frequency is exactly 10 kc. At the slowerspeeds the pilot frequency developed by the tachometer will besuccessive binary submultiplies of the piiot frequency, 5 kc., 21/2 kc.,etc., down to 0.625 kc. at the 15 inch per second tape speed.

The pilot signal, of whatever value, is fed through a switch 19, to anamplifier 21, the output of the amplifier connecting to a tachometerlead 23. This same signal is also supplied through a switch 25 to atransducer head 27, which records the same signal upon the tape. It maybe noted at this time that switches 19 and 25 are preferablymechanically interconnected or ganged so that in the playback positionthe photocell circuit is disconnected from the amplifier 21 and thetransducer head disconnected from the amplifier output and connected toits input. The transducer head therefore picks up the pilot frequency onplayback and supplies it to the tachometer lead 23. As in a properlydesigned tape drive no measurable slip occurs between the tape and thecapstan that drives it, the pilot signal in reproduction is stilldirectly proportional to the motor speed and, in effect, a tachometersignal. Owing to changes in dimension of the tape through variations intemperature, humidity or other less obvious factors there may be veryslight differences in motor speed during recording and playback inrecording and reproducing the same frequency. The pilot signal will,however, always bear the proper relationship to the other signals thatmay be recorded on the tape, playingl back the recorded signals at theiroriginal value.

The motor control is exercised through the comparison of the pilotsignal with standard or comparison signals of the same frequency. Todevelop these signals a stabilized oscillator 29 is used. Thisoscillator may, for example, be a crystal controlled oscillatoroperating at 40 lic.: i.e., at four times the highest frequency at whichthe tachometer is to be stabilized. In order to minimize the load uponthe oscillator it connects through a buffer amplifier 3l to a cascade ofbinary frequency dividers designated'as 331, 332, 333 and 334. Theaccuracy of control is increased if the final stage of the bufferamplifier is over-driven so that it delivers a substantially squarewaveform to the frequency dividers. The input and output terminals ofeach of the frequency dividers connect to the various contacts of amulti-point switch 35 by means of which it is possible to select eitherthe 4() kc. frequency developed directly by the standard frequencyoscillator or any of its binary submultiples down to 2.5 kc. l Y

As is customary in binary frequency division, the frequency dividersused are preferably bistable'multivibrators or Eccles-Jordan flip-hops.Switch 35 connects the signals developed at the terminals of anyselected one of Vthese flip-Hops to the input of a similar frequencydivider 37. There are available at the output of this multivibratorsquare waves of half the input frequency and of opposite phase. Both ofthese waves are used; one, designated as Zero phase connects to afurther multivibrator designated as 39C, the other, of 180 phase, issupplied to a similar divider 39S. The result is that in the output ofthis pair of frequency dividers there appear two square waves of thesame frequency, the fundamental components whereof are displaced inphase by 90 degrees and therefore the output circuits of the dividerscan be conveniently referred to as the sine circuit 41 and the cosinecircuit 43. It may be noted that if the apparatus were intended tooperate at a single speed only, any method of developing sine and cosinewaves could be used. 'Ihese would not necessarily have to be squarewaves, although, as will be shown, there is marked advantage in thesquare waveform. The advantage of the arrangement shown is that the twostandard frequency waves developed by it are accurately displaced by 90degrees irrespective of the frequency at which they are operated. j

The various phase relationships are shown in the waveform diagrams ofFIG. 2. In this gure curve A illustrates the waveform delivered at pointA in FIG. l, i.e., the input of the divider 37. Waveform B is that atpoint B and waveform IC that at point C, the inputs to voltage divider39C and 39s respectively. In each case it is assumed that all of thefrequency dividers are designed to trigger on the rise of the input waveand that the risetime of each of these is so short in comparison withthe periods of the .waves that the rise can be considered instantaneous.output of frequency divider 39S and E that of the output of 39C. Acommon time base is employed for all of the waveform curves of thisfigure to indicate the phase relationships between the waves at thesuccessive points of the circuits.

Waveform D, in the sine circuit, is applied through a series resistor45s to a shunt condenser 47S. Resistor and condenser combine to form anintegrating circuit, beyond which a blocking condenser 49s is insertedin series in the sine circuit. With this arrangement, since there is noleak to ground across condenser 47s, its charge is determined wholly bythe currents supplied to it from the frequency divider. The result isthat the square waveform is integrated into a substantially puretriangular waveform which appears at point F in the circuit and isillustrated in the curve F of FIG. 2.

A similar integrating circuit, the elements whereof are designated bythe same reference characters as those in the sine circuit butdistinguished by the subscripts c, is included in the cosine circuit,thus developing at the point G, the input to condenser 49e, awaveformshown inthe In like manner waveform D is that of the curve G of FIG. 2.Curves F and G are superposed, curve G being dotted in order toillustrate the relative phase relationships between these two curves.

Beyond the blocking condensers 49 the sine and cosine circuits connectto electronic switches, illustrated symbolically at Slls and 51C. Theseswitches operate to close the circuit connected to their respectiveoutputs only when a voltage of proper sign is applied to a controlcircuit. Many such switches have been devised. The ordinary type of ringmodulator, familiar in telephone carrier circuits, will operate in thismanner provided one of its inputs is supplied with unidirectionalpulses. Another such switch is shown in Waveforms, vol. 19, RadiationLaboratories Series (McGraw-Hill 1949), page 374. rll'his latter switchis the one employed in the circuit here described, but this is a matterof engineering choice. ri`he characteristic required of the switch usedis that it be bidirectional; i.e., that it pass current of eitherpositive or negative polarity during the actuating pulse and open thecircuit between its input and its output in the absence of the controlvoltage.

The outputs of the two switches connect to small shunt storagecondensers 53S and 53C. It may be noted that no conductor or leak-pathis provided across the storage condensers 53. Accordingly, when theswitches 5l are closed, the condensers charge to the potential presentat that instant in the sine or cosine signal, as the case may be, andretain that charge until the next closure of the switch.

The control voltage for closing the switches 51 is applied from thetachometer line 23. The invention will operate directly from thepositive (or negative) halfcycles of the pilot signal, but its accuracyof control is greatly increased by developing unidirectional pulseswhose duration is short in comparison with the period 0f the pilot waveand which occur at a constant phase of the pilot. Accordingly, there ispreferably inserted in the pilot signal lead a pulse generator 55, whichconverts the substantially sinusoidal wave into a train of suchshort,'unidirectonal pulses. As in the case of the switches, the pulsegenerator may take any of several forms. One such form is a clippingcircuit that passes only the extreme peaks of the positive half waves,e.g., a back-biased diode or a Zener diode. Another form, frequentlyused, is a differentiating circuit which generates a pulse when thepilot wave is changing most rapidly, as it crosses the zero axis. Ifthis latter form is used both positive and negative pulses aregenerated. It so happens that with neither of the forms of switches thathave been specifically mentioned is it necessary to suppress a negativepulse, although in other forms of switch this may be desirable. Wherering modulator is used as a switch a positive pulse sampling a positivewave will deliver a positive output, while a negative pulse, occurringone-half cycle later, will sample a wave in negative phase and alsoyield a positive output, so that the only effect is to take twice asmany samples as where only the positive pulse is used. The type ofswitch shown in the waveforms citation given above is responsive topulses of one sign only; while the actuating pulses may be of eitherpolarity they are here shown as positive with respect to the A.C. axis.

Curve H of FIG. 2 indicates the position of the pulses in respect towaves F and G at approximately the position at which they occur whenstable operation of the apparatus has been established., it will benoted that it samples the sine wave very near to the point where itcrosses the zero axis and hence the charge on the storage condenser 53sis small, whether positive or negative.

VThe same sampling instant, however, corresponds with very nearly themaximum of the cosine wave and a maximum charge is therefore applied tostorage condenser 53C.

Beyond the storage condensers 53 the sine and cosine circuits are nolonger symmetrical. In normal operation, after synchronous operation hasbeen established, the .control is exercised primarily by the sinecircuit. This circuitk forms a portion of a high-gain feedback loop, andwhile after stable operation is established its operation issubstantially on direct current, during acceleration, until the motorreaches pull-in speed, it carries alternating components of relativelyhigh amplitude. Since as has already been mentioned, the samplingoperation performed by the switches Sl is essentially a modulationprocess, the frequencies developed include as nominal components, thesum and diiference frequencies of the pilot and standard-frequencywaves. Within the range of frequencies thus developed, some phase-shiftwill occur in the motor circuits and with the high gain that isdesirable the amplifier may become unstable at. extreme frequencies. Tocompensate for such phaseshift it is therefore preferable to include alead-lag network comprising a series resistor 55 of, say, about 1 megohmvalue, shunted by a 0.02 microfarad condenser, followed by shunt armcomprising a 100 kilohm resistor 59 in series with a l microfaradcondenser 6l. The use of such lead-lag networks is conventional and itwill be understood that the specic values given are those appropriate tothe particular circuit elements later to be described. Other elements orother frequency ranges might require different constants but those givenare typical.

Following the lead-lag network the sine circuit connects to adirect-coupled, DC. amplifier of variable gain, depending on themagnitude of a control voltage. ln the present instance the first stageof this amplifier is the pentode section 62 of a dual SUSpentode-triode. The cathode 63 of this tube connects to a 150 v. supplythrough a 3 kilohm cathode resistor 65, bridged by a by-pass condenser67. rl`he control grid 69 connects directly to the junction between theseries and shunt arms of the lead-lag network and ti e screen grid 7l isgrounded. The plate connects to a positive supply through an anoderesistor '73. In order to establish the proper relative operatingvoltage between the control grid and the cathode, a lead from the 150 v.source connects through a 100 kilohm resistor 75 to the input side ofthe switch Slis. This establishes the initial potential of the condenser53S at the 150 V. level; the alternating components supplied through theblocking condenser 495 therefore serve to charge and discharge thiscondenser with respect to 150 v. as the norm, being superposed withrespect to ground, upon the l5() v. negative bias.

The anode 77 of the pentode section is directly connected to the controlgrid 79 of the triode section SG of the tube. The triode section isoperated as a cathode follower', the anode ill connecting directly to a+5() v. source, the cathode connecting back to the -150 v. sourcethrough cathode resistor 83.

rlhe voltage across the cathode resistor is applied directly to thecontrol grid SS of a pentode that controls the gain of the ampiiier andis, in fact, a modulator. The cathode of this tube operates at groundpotential; the screen grid 37 operates at approximately +150 v. whileits anode S9 connects to a +3007 v. source through a load resistor 9i.The suppressor grid 93 is supplied from the cosine circuit that willnext be described.

The cosine circuit is AJC. connected and comprises an amplifier usingthe same type of dual tube (a 6U8) as is used in the sine circuit. Asused in the cosine circuit, however, the control grid of the pentodesection 9d of this tube connects directly to storage condenser 53C andits normal operating point is established at ground potential by thegrid resistor 95, connected, as is the case in the sine circuit, aheadof the sampling switch. The tube is biased through cathode resistor 97,connected to ground. Anode current is supplied through the usualresistor 99 from the +300 v. source.

The triode section lill of the dual tube is coupled to the pentodesection through a differentiating network cornprising a series condenserN3 and a shunt resistance network including a resistor 105, thatconnects directly to ground, and a second resistor 07 that connects tothe v. source. It will be seen that these two rcsistors are effectivelyin parallel for the A.C. components. The control grid of the triodesection connects to the junction of the two resistors which act as avoltage divider to apply the proper operating potential to the grid. Thetube is connected as a cathode follower, with its output circuitcomprising cathode resistor 169. The cathode connects to the suppressorgrid 93 of the modulator tube as previously mentioned. The output of themodulator connects to the input of the conventional D.C. amplifier '7that supplies armature current to the drive motor.

ln describing the operation of the apparatus it is convenient toconsider first the normal running condition, where the motor speed hasstabilized so that the frequency of the pilot signal is equal to thestandard frequency in the sine and cosine circuits. When this is thecase the sampling pulses always occur at very nearly the same phase ofthe standard frequency cycle, it being understood that for conveniencethe phase of the triangular wave means the phase of the fundamentalfrequency thereof.

As shown by curves F, G and H of FIG. 2, at the stabilzing point thesampling occurs very near the zero aXis of the sine wave and very nearthe negative peak of the cosine wave, for the highest tape speed, withthe preferred adjustment of the apparatus; for lower speeds samplingoccurs farther to the left. If it should happen that the variousamplifier stages in the cascade supplying the motor armature have theiroperating points so chosen that no signal voltage on condenser' 53sresults in a DC. output to the armature just suticient to operate themotor at the desired speed, the sampling would take place at exactlyzero of the sine wave. It is highly unlikely that this precise situationwill be met; it certainly will not ybe met at all tape speeds, for theload on the motor will obviously be much less at l5 inches per secondthan at 240 inches. The operating point of the amplifier will preferablybe set approximately correctly for an armature speed, somewhat above thehighest required, so that with zero signal voltage on condenser 53s themotor would run somewhat too fast for 240 operation.

As a result, when the apparatus has reached synchronism, there is aconstant tendency for the armature to pull ahead of the desired speed,making the sampling point come earlier in successive cycles and move thepulse, as illustrated in FIG. 2, farther to the left. This results in anincreasing negative charge on condenser 53s, decreasing the currentsupply to the armature, and consequently the motor torque and the speed.The sampling point, therefore, quickly stabilizes at about the pointshown in the ligure, assuming the switch 35 to be set for the maximumtape speed. For lower speeds the sampling point will be still farther tothe left.

Where only a single tape speed is required the operating points of thevarious amplifiers can be set so that with no charge on condenser 53sthe armature current is either higher, lower or equal to the currentrequired at synchronous speed. The setting should always be greater thanthat required for one-half the desired speed, since under theseconditions lock-in can occur on a sub-harmonic of the standardfrequency. Maximum current at zero signal has been found to prevent thisand as this latter setting of the operating point also gives maximumstarting-torque and quickest lock-in it is always to be preferred, eventhough it is not a necessary condition for operation.

By postulate the D Campliiier has a high gain and a relatively smallsignal voltage on condenser 53s will make a relatively large change inarmature current. Accordingly, at high tape speeds the sampling pulsesfrom the pilot signal will lag, moving the pulse H to the right in thefigure and charging condenser 53s to a positive potential whichincreases the current through the pentode section of tube 62. Thisdrives the grid 79 of the triode section negative, the cathode also goesnegative, decreasing the current in the modulator tube and driving theinput of the D C.v amplifier 7 positive to increase the armaturecurrent. At lower tape speeds the pilot signal tends to lead thestandard frequency moving the sampling frequency to the negative sideofthe A.C. axis reducing the armature speed to proper value.

Where the sine component is sampled at near zero the simultaneoussampling of the cosine component occurs near the negative maximum. Thisresults in minimum current through pentode section 94 but because thesampling occurs at substantially constant phase of the standardfrequency waves the charge on condenser 53C remainssubstantiallyconstant, without any material AC. component. 'Sincethecathode follower stage lill is A C. coupled to the circuit theresultis that no signal is suplied to suppressor grid 93 of themodulating tube, which accordingly operates at substantially constantpotential, in'this case near but positive to the cathode potential,

In practice, ofcourse, such a completely static situation as regards thechargeson condensers 53 does not occur; there will be some hunting, verysmall, about the average speed. This hunting tends to be sinusoidal innature, moving the sampling point up and down the slope of the curve Fto develop essentially a sine wave in the output of both condensers.Actually, of course, the output will be a stepped wave, because of theintermittent sampling, but as the hunting that does occur is normally atlow frequency, somewhere inthe order of 20 cycles per second, at maximumtape speed, so many samples are taken in each cycle of the hunting thatthe resulting A.C. components can be taken as sinusoidal Without anymaterial error. The phases of the sinusoidal signals thus developed inthe sine and cosineV circuits are in the same relation as the phases ofthe respective standard frequency waves in these circuits; i.e., thephase of the signal in the cosine circuit lags 9G degrees that in thesine circuit. The cosine signal is supplied to the cathode followersection lill, however, through a differentiating network, which advancesthe phase of the cosine signal by 90 degrees and brings it into phasewith that in the sine circuit. A modulator tube is a multiplier,supplying these alternating components iii-phase to the control andsuppressor grids respectively of the multiplier tube. Since they changesign, from positive to negative, simultaneously, the product is always apositive number during stablestate operation. Therefore, the net effectof the cosine component multiplied by the sine component is to increasethe amplification of the modulating tube so that the cosine componentaids the sine component in control.

Although the effect of the differentiating circuit is always to advancethe phase of the alternating components by 90 degrees, the amplitude ofthe differential signal is not constant but is directly proportional tofrequency. as a result the control exercised by the cosine circuit isnegligible for very slow hunting rates while it becomes greater at rapidhunting rates. In the control of hunting, under stabilized, runningconditions, the effect of the cosine signal is small, but what effect it1nas is in the proper direction to give good control.

The primary purpose of the cosine circuit is in starting the motor andpulling it into phase. At the instant of closing the motor switch thereis, of course, no pilot signal to sample the standard wave in eithercircuit. The current supplied from the D.C. amplifier at this firstinstant is the normal running current as determined by the operatingpoints of the D C. amplifier, as is the case when the sampling occurs atzero of the sine wave. As soon as rotation starts, however, samplingstarts to occur. Owing to the step nature of the sampled waves and therapidly changing conditions, an exact analysis of theinter-relationships between the koutputsof the two control circuits asapplied to the modulator tube is somewhat diicult. Since sampling is aform of modulation, however, the principal components normally presentin a modulated output do occur, but with changing amplitude as therelative frequencies approach each other. At the start of the operationthe sampling frequency dominates, but as synchronism is approached thedominant frequency in both outputs is the difference component. Underall circumstances, however, the sine' and cousine relationship ismaintained between the modulation products in the two circuits and dueto the advance in phase resulting from the differentiating circuitsupplying cathode follower 101, the effect is always to increase theoutput of the modulator tube up to the point when synchronism occurs, ashas already been described in connection with the control of hunting.

As synchronism is approached, the waveform of the output of bothcircuits approaches the triangular waveform or" the standard frequencywaves, occurring at the difference frequency, but stepped at thesampling frequency, and always in the direction to assist the pull-in.

While, in ordinary operation, the function of the cosine circuit is toassist in speeding up the motor to the pull-in point, there may beconditions where it is necessary to slow it down. Thus, for example, theapparatus might be started initially with the switch 35 connected to runthe tape at high speed, say 240 inches per second, when what wasactually desired was a recording at a lower speed, say 60 inches persecond. In this case the pilot frequency is greater instead of less thanthe standard frequency. The difference frequency is still the dominantfrequency in the output of the sine and cosine circuits. In the sinecircuit however, the difference component changes sign when the samplingfrequency becomes greater than the standard Ifrequency, while the cosinefunction does not change sign. The phase advance in the differentiatingnetwork therefore brings the difference frequency componentsout-of-phase, as applied to the control and suppressor grids of themodulator tube, instead of in-phase and as a result the product of thetwo waves as applied to these grids is always a negative number, tendingto decrease the armature current instead of increase it, and againpulling the motor into step where the sine circuit can take over itsnormal control.

It should be apparent from the foregoing that the apparatus will operatewith sine-wave instead of squarewave control signals, since thereasoning that has been applied has referred primarily tothe fundamentalcomponents of all of the waves. This has been done for the purpose ofsimplicity only, since the factors discussed are generally true asregards all components considered separately. The advantage of usingvtriangular waves arises from the fact that the range through whichcontrol by the sine circuit can be exercised is much greater, coveringphase differences of a complete half cycle instead of only a few degreeson either side of the zero point, where the rate of change of the sinewave is greatest. By proper amplification the slope of the entire halfcycle of the wave may be made as great as the steep portion of acorresponding sine wave.

The sensitivity and speed of response of the feedback loop are veryhigh. it will respond to deviations in motor speeds occurring at ratesas high as 60 cycles per second; this is the fastest rotational rate ofthe motor used and the circuit is therefore more than ample to take careof any normal hunting. At lower speeds it is even able to compensate forsome capstan eccentricity, although in any propely constructed apparatusthis is the least important source of error.

With a properly constructed drive, employing a truly concentric capstan,it has been found that the maximum hunting that occurs at the 240 inchtape speed, correspending to a l0 kc. standard frequency is i0.lmicrosecond. This is 1,40 of one percent of a pilot frequency cycle orabout 7 seconds of arc of the armature position.

anos,

i This is comparable to the best that can be obtained using synchronousmotors.

it will be recognized by those skilled in the ar-t that the actualorganization of apparatus shown and described can be modified in manyways and still perform the required functions. As described, a positivevoltage on condenser 53S increases the armature current; one more or oneless anode-coupled stage in the DC. amplifier would cause such apositive voltage to decrease armature current butthe operation would beexactly the same except that stabilization would occur when the samplingpulse took place one-half cycle displaced from that shown in the curves;on the descending side of curve F instead of the ascending side. Thephase relationships can be juggled almost at will provided the signalsappear in the proper phase, relative to each other, in the devicecontrolling the gain of the DC. amplifier. Thus, for example, the cosinewave is described as such because it lags the waves in the sine circuitby 9() degrees. If it were reversed in phase, i.e., if the wave weretaken from the other terminal of frequency divider 39C it would lead by90 degrees. if, however, the sampling pulse were applied to a ringmodulator as switch Sie also in reverse phase, the double reversal wouldresult in bringing back the relationships that have been explained indetail. The criteria are that the waves are displaced by 90 and that theresultant waves as applied to the modulator yield a number of the propersign when multiplied so that if the pilot frequency is lower than thestandard frequency the overall amplification is increased and if thepilot frequency is the higher it is decreased. Beyond this the termssine and cosine are for identification only.

Another example of flexibility of design is the final modulator whereinthe sine component and the 90 degree-advanced cosine component arecombined. Any type of modulator that will multiply the plus sinecomponent -by the plus-or-minus sine component is satisfactory and itwill be realized that this describes substantially any amplitudemodulator.

The organization of apparatus shown is therefore intended to beillustrative only and not as limiting the scope of the invention, allintended limitations being expressly set forth in the claims thatfollow.

I claim:

l. A constant-speed drive for recording mechanisms and the likecomprising a shunt-type DC. motor, means driven by said motor forgenerating a pilot wave the frequency whereof varies directly with therotational speed of said motor, and means for locking the rotationalspeed of said motor at a desired value giving a fixed frequency fromsaid generating means comprising means for developing two electricalwaves of said fixed frequency the fundamental components whereof are inphase quadrature and supplying said electrical waves to a sine wavecircuit and a cosine wave circuit respectively, means actuated by saidpilot wave for simultaneously sampling the instanltaneous magnitudes ofthe waves in said sine and cosine circuits, condensers connected inshunt in each of said circuits for storing charges proportional to saidsamples between sampling intervals, D.C. amplifying means responsive tothe magnitude of the charge on the condenser in said sine circuitconnected to supply operating current to one of the shunt circuits ofsaid D.C. motor, means connected in said cosine circuit for advancingthe phase of any alternating component in the charge of the condenserconnected therein, and modulating means interconnecting said sine andcosine circuits to control the output of said D.C. amplifying means inproportion to the product of the outputs of said sine and cosinecircuits.

2. A constant-speed drive as defined in claim 1 wherein said means fordeveloping two electrical waves develops waves of triangular wave-form.

3. A constant-speed drive as defined in claim l wherein said samplingmeans includes means for deriving from said pilot wave a train of pulsesrecurring `at pilot-wave frequency and switching means operative toclose said l2 sine and cosine circuits respectively to said condensersduring said pulses only.

4. A constant-speed drive for recording mechanisms and the likecomprising a shunt-type D.C. motor and means driven by said motor forgenerating a pilot wave of a frequency directly proportional to therotational speed of said motor, and means for locking the rotationalspeed of said motor at a desired value giving a fixed frequency of saidpilot wave comprising `a source of electrical oscillations of 4 timessaid fixed frequency, a first binary frequency divider driven by saidsource, a pair of binary frequency dividers connected to be driven byopposite phases of said first frequency divider and each adapted toprovide a substantially square-wave output, a sine wave circuit and acosine wave circuit connected respectively to one of said pair offrequency dividers and each including, in succession, integrating meansfor converting the square waves therein to triangular waves, anelectronic switch and a storage condenser connected to be charged to thepotential of t re circuit to which it is connected when said electronicswitches are closed; a variable-gain DC. amplifier connected from thestorage condenser in said sine circuit to supply current to one of theshunt circuits of said D.C. motor, an A.C. amplifier connected from thestorage condenser in said cosine circuit and including a differentiatingnetwork for advancing the phase of A.C. components therein, connectionsfor supplying said components to control the gain of said D.C. ampliher;and connections from said pilot-wave generating means to both of saidelectronic switches to close said switches simultaneously during alimited portion of each cycle of said pilot wave.

5. A `drive mechanism as defined in claim 4 including means in saidconnections from said pilot wave generatmeans for developing from saidpilot wave a train 0f sharp pulses of pilot-wave repetition frequencyfor actuating said electronic switches.

6. A drive mechanism as defined in claim 4 for operating recordingmechanisms and the like at any selected one of a plurality of integrallyrelated constant speeds wherein said source of oscillations comprises anoscillator operative at an integral of all of the fixed frequenciesdeveloped by said pilot wave generator at desired rotational speeds andat least 4 times said fixed frequency at the highest of said fixedspeeds, a plurality of frequency dividers connected in cascade to saidoscillator, and switching means for connecting said first frequencydivider recited in claim 4 optionally to the terminals of any of saidcascade-connected frequency dividers.

7. In combination for controlling the speed of a motor in driving amedium capable of recording and reproducing information, means coupledto the motor for generating a pilot signal having a frequency related tothe speed of the motor, means for generating `a standard signal at aparticular frequency, means responsive to the pilot and standard signalsfor generating a first alternating signal having characteristics relatedto any difference between the frequencies of the pilot and standardsignals, means responsive to the pilot and standard signals forgenerating a second alternating signal having characteristics related toany difference between the frequencies of the pilot and standard signalsand having, relative to the first alternating signal, a polaritydependent upon the relative frequencies of the pilot and standardsignals, and means responsive to the first and second alternatingsignals for regulating the speed of the motor.

8. yln combination for controlling the speed of a motor in driving amedium capable of recording and reproducing information, electricalcircuitry responsive to the speed of the motor for providing pilotpulses at a rate related to the speed of the motor, electrical circuitryfor providing a standard signal at a particular frequency, electricalcircuitry including first gating means responsive to the pilot pulsesand the standard signals for producing a first alternating signal havingan amplitude dependent upon the frequency of the standard signals andthe rateof the pilot pulses, electrical circuitry including secondgating means responsive to the pilot pulses and the standard signals forproducing a second alternating signal having an amplitude dependent uponthe change of any difference between the standard signals and the pilotpulses and having, relative to the first Aalternating signal, a polaritydependent upon the relative frequencies of the standard signal and thepilot pulses, and electrical circuitry responsive to the first andsecond alternating signals for combining said first and said secondalternating signals for producing signals for introduction to the motorand having characteristics for regulating the speed of the motor.

9. in combination for controlling the speed of a motor in driving amedium capable of recording and reproducing information, meansresponsive to the speed of the motor for generating pilot signals at afrequency related to the speed of the motor, means for generating twodifferent standard signals at the same particular frequency, firstcontrol 4means responsive to one of the two standard signals and to thepilot signals for .generating first control signals havingcharacteristics to maintain the speedl of .the motor at a particularrelationship to the frer v quency of therstandard signals, secondcontrol means responsive to the other of the two standard signals and tothe pilot signals for generating second control signals havingcharacteristics to obtain a quick acceleration of the motor to thedesired speedfrom a starting position, and means responsive to the firstand second control signals for combining the first and the secondcontrol signals to produce signals for introduction to the motor andhaving characteristics for regulating .the speed of the motor.

l0. :In combination for controlling the speed of a motor in driving amedium capable of recording and reproducing information, electricalcircuitry responsive to the speed of the motor for producing a pilotsignal having a frequency related to the speed of the motor, electricalcircuitry for producing a standard signal at a substantially constantfrequency, electrical circuitry responsive to the standard and pilotsignals for producing a first control signal having characteristicsrelated to the difference in frequencies between the standard and pilotsignals, electrical circuitry responsive to the standard and pilotsignals for producing a second control signal having characteristicsdependent upon the relative frequencies of the pilot and standardsignals, and electrical circuitry responsive to the first and secondcontrol signals for combining the first and the second control signalslto generate a signal for introduction to the motor to regulate thespeed of the motor.

l1. In combination for controlling the speed of a Imotor in driving amedium capable of recording and reproducing information, electricalcircuitry for providing standard signals at a particular frequency,electrical circuitry responsive to the signals at the standard frequencyfor producing a pair of signals at the standard frequency with aparticular phase relationship to each other, electrical circuitryresponsive to the speed of the motor for lproducing pilot signals havinga frequency related to the speed of the motor, electrical circuitryresponsive to the pilot signals and to one of the standard signals inthe pair for producing a first alternating signal having characteristicsrelated to any differences between the frequencies of the standard andpilot frequencies, electrical circuitry responsive to the pilot signalsand to the other of the standard signals in the pair for producing -asecond alternating signal having characteristics related to thefrequencies of the pilot and standard signals and having, relative tothe first alternating signal, a polarity dependent upon the relativefrequencies of the pilot and standard signals, and electrical circuitryincluding a direct current amplifier responsive to the first and secondalternating signals for combining the first and second alternatingsignals to produce a control varying direct current signal forintroduction to the motor and having characteristics for regulating thespeed of the motor.

l2. In combination for controlling the speed of a mot-or in driving amedium capable of recording and reproducing informaton, means forproviding a pair of standard signals both at a first frequency but atdifferent phases, means responsive to the motor for providing pilotpulses at a rate related to the speed of the motor, means includingfirst energy-storage means responsive to one of the pair of standardsignals and to the pilot pulses for storing in the energy-storage meansan amount of energy dependent upon the relative frequencies of one ofthe standard signals and the pilot pulses, means including secondenergy-storage means responsive to the other one of the pair of standardsignals and to the pilot pulses for storing in the second energ -storagemeans an amount of energy dependent upon the relative frequencies ofthev other of the standard signals and the pilot pulses, and meansincluding control means responsive to the energy levels in the firststorage means and in the second storage means to produce a signal forintroduction to the motor and'having characteristics for regulating thespeed of the motor.

13. In combination for controlling the speed of a motor in driving amedium capable of recording and reproducing information, electricalcircuitry responsive to the speed of the motor for producing pilotpulses at a rate related to the speed of the motor, electrical circuitryfor producing first standard signals having a particular frequency,electrical circuitry responsive to the standard signals for producingsecond and third standard signals having a frequency related to thefrequency of the first standard signals and having a particular phaserelationship to each other, electrical circuitry including firstswitching means responsive to the pilot pulses and to the secondstandard signals and including first energystorage means coupledelectrically to the first switching means for producing a first controlsignal having characteristics dependent upon the rate of the pilotpulses relative to the frequency of the first signals, electricalcircuitry including second switching means responsive to the pilotpulses and to the third standard signals and including secondenergy-storage means coupled electrically to the second switching meansfor producing second control signals having characteristics dependentupon the rate of the pilot pulses relative to the frequency of the firstsignals, and electrical circuitry including a modulator responsive tothe first and second control signals for producing a voltage forintroduction to the motor and having characteristics for regulating thespeed of the motor.

14. In combination for controlling the speed of a motor in driving amedium capable of reading and recording information, means for providingfirst and second standard signals each having the same particularfrequency and each having a particular different phase relative to theother signal and each having a wave form progressively varying betweenfirst and second amplitudes, means responsive to the speed of the motorand to the first standard signal for reproducing the amplitude of thefirst standard signal at particular instants in successive cycles of thefirst standard signal in accordance with the speed of the motor relativeto the frequency of the rst standard signal, means responsive to thespeed of the motor and to the second standard signal for reproducing theamplitude of the second standard signal at the particular instants inaccordance with the speed of the motor relative to the frequency of thesecond standard signal, and means responsive to the reproducedamplitudes of the first and second standard signals for combining thereproduced amplitudes of the first and second standard signals toproduce a control signal having characteristics for regulating the speedof the motor.

15. In combination for controlling the speed of a motor in driving amedium capable of reading and recording information, means includingfirst electrical circuitry for providing first and second standardsignals each having a particular frequency and each having a particulardifferent phase relative to the Aother and each having a triangularconguration in successive half cycles, means including second electricalcircuitry responsive to the speed of the motor and to the first standardSignals and including first energy-storage means for storing in theenergystorage means an amount of energy related to the amplitude of thefirst standard signal at particular instants in successive cycles and inaccordance with the speed of the motor relative to the frequency of thelirst standard signals, means including third electrical circuitryresponsive to the speed of the motor and to the second standard signalsand including second energy-storage means'for storing in theenergy-storage means an amount of energy related to the amplitude of thesecond standard signal at the particular instants and in accordance withthe speed of the motor relative to the frequencies of the secondstandard signals, and means including fourth electrical circuitryresponsive to the energy levels in the first and second storage meansfor producing a control signal having characteristics yfor regulatingthe speed or' the motor.

16. In combination for controlling the speed of a motor, means driven bythe motor for generating a pilot wave the frequency whereof variesdirectly with the rotational speed of the motor, means for developingtwo refiti erence electrical waves of xed frequency and in phasequadrature with each other, -rst means coupled to said generating meansand to said developing means for providing a rst control signal inaccordance with the difference offrequencies between the pilot wave andone of the two reference electrical waves, second means coupled to saidgenerating means and to said developing means for providing a secondcontrol signal in accordance with the difference of the 4frequenciesbetween the pilot `wave and the other one of the two referenceelectrical waves, means coupled to said second means for advancing thephase of any alternating component of the second control signal, anddirect current amplifying means coupled to said irst means and to saidphase advancing means for amplifying the first control signal and thephase advanced component of the second control signal to provide acomposite control signal for introduction to the motor for controllingthe speed of the motor.

References Cited in the tile of this patent UNITED STATES PATENTS2,749,496 Newman et al June 5, 1956 2,782,355 Wilcox Feb. 19, 1957

